The turbine blades of steam turbines are often provided with a cover plate in each case integrally formed on the head side on the blade leaf. Normally, the turbine blades combined in each case into moving blade rows or guide blade rows are arranged on the rotor or on the casing of the steam turbine in such a way that the whole of the cover plates of a blade row which in each case project laterally beyond the blade leaf forms a continuous ring, what is known as a shroud. In this case, the cover plates of the turbine blades assigned to a blade row are, as a rule, keyed or clamped with respect to one another during installation, in such a way that further fastening means or connection elements between the individual cover plates can be dispensed with. By the turbine blades being coupled in the annular shroud, vibrations or distortions of individual turbine blades which occur as a result of high dynamic stress are effectively suppressed.
The cover plates or a blade row in each case combined into a shroud are designed to minimize the gap and edge losses caused by a secondary flow over the blade tips or the shroud. For this purpose, particularly when the steam turbine is operating under full load, as small a gap width as possible is desired between the shroud and the casing or rotor lying opposite it. On the other hand, brushing during operation should as far as possible be avoided. Particularly during unsteady operating actions, that is to say, for example, during starting and in the event of load changes, however, there is the risk of comparatively pronounced relative length changes of the components involved which are caused by different thermal expansion, so that, in exceptional cases, brushing still has to be reckoned on. In order to keep the expansion of potential contact points as low as possible and therefore also minimize the frictional forces occurring in the event of contact, metal strips or metal rings, what are known as sealing bands, fastened to the shroud or to the casing or rotor lying opposite it and running in the circumferential direction are used. When rotating and stationary parts come nearer to one another than planned, first the comparatively thin sealing bands in this case come into contact with the opposite component, the surfaces of the two contact partners grinding against one another in a usually locally limited wearing region. This ensures sufficient emergency running properties, at least in the case of once-only or brief brushing.
If operating states of this type occur more frequently, however, there is, if the sealing bands are attached in each case to the turbine component lying opposite the shroud, that is to say to the casing (in the case of a moving blade shroud) or to the rotor (in the case of a guide blade shroud), the risk of continued wear which damages the cover plates or the shroud as a whole. In this case, under certain circumstances, the sealing band may “pit” deeply into the shroud, which after some time may even lead to an almost complete removal of the shroud. The stability of the originally annularly closed composite cover plate structure is considerably diminished due to wear-induced local interruptions, this being conducive to the occurrence of blade oscillations. Moreover, in the case of continued wear or excessively high oscillation amplitudes, fragments of macroscopic size or even whole turbine blades may come loose and then be thrown with high momentum against the turbine blades or casing parts of the following turbine stages. In an extreme case, this may lead to a complete destruction of the steam turbine.
FR-A-1 470 032 discloses turbine blades with shrouds.
US 2003/107181 A1 discloses a seal between a stationary and a movable part, the immovable part having an abrasive layer and the movable part being arranged touch-near to this abrasive layer.
EP 1 312 760 discloses a turbine blade tip with an abrasive surface, the abrasive surface comprising abrasive particles.
US 2003/183529 A1 discloses abrasive layers with a high oxidation resistance.